Fretting wear is one type of wear. This fretting wear indicates the wear when sliding or vibrating surfaces of fastened parts of a structures abrade due to fine vibration of the sliding or vibrating surfaces of the structures in the state where the sliding or vibrating surfaces of the structures continuously receive fine vibration while receiving pressure. Therefore, it is difficult to judge the state of advance of this fretting wear from the appearance of these structures. Further, due to the differences in the environments in which the fastened parts of these structures are placed, the degree of wear of the fretting wear often differs. Usually, fastened parts of structures are retightened based on the state of use of the structures, but fretting wear has little effect. Depending on the state of advance of fretting wear of structures, this leads to the structures themselves fracturing.
Locations of structures susceptible to fretting wear are locations where the structures are pressed against each other by a strong pressure, that is, sliding or vibrating surfaces of the structures with strong fastening force. Furthermore, it is known that when members of structures receive fine vibration, regardless of whether the direction of this sliding or vibration is a direction parallel to or vertical to the sliding or vibrating surfaces or a composite of the same, wear occurs particularly easily inside the fastening force lines (the range affected by the force fastening the head of a bolt with the nut represented by the lines).
If giving an example where fretting wear easily occurs, structures such as automobiles and other vehicles receive sliding or vibration from the ground continuously while running. The sliding or vibration can also be called “fine vibration”. In automobiles and other vehicles, the sliding or vibration from the ground is transmitted to a passenger through the tires, tire wheels, hubs, chassis, suspension, shock absorbers, body, cabin, seat, etc. in that order. In automobiles and other vehicles, the tires absorb vibration, but most of the vibration is transmitted from the tire wheels to the hubs and brake drums. Further, an automobile runs utilizing force generated by the engine, so vibration of the engine etc. is transmitted to the drive system and further transmitted through the hubs to the tire wheels, tires, and ground. The tire wheels and hubs are structures assembled by fastening members. The fastening members of the tire wheels and hubs receive vibration of the tires, the weight of the body, and the load at the time of braking. Further, in an automobile or other vehicle etc., when turning, the steering wheel is turned and the hubs are made to face the direction of advance through the steering mechanism.
As a result, the tire wheels are also turned to the braking system side, but the vertical surfaces of the hubs also are acted on by forces by which they are turned to the braking system side. The fastened contact surfaces of the tire wheels and hubs rub against each other and fine vibration is constantly given.
In recent years, to deal with the increased weight of vehicles, efforts are being made to lighten the parts. Vehicles are increasingly being equipped with tire wheels made by aluminum (A6061). Such aluminum tire wheels oxidize at their surfaces resulting in the formation of oxide film. When part of this oxide film peels off and the base material is exposed, it further bonds with the oxygen in the air resulting in the formation of new oxide film. This oxidized aluminum is a substance called “alumina” (Al2O3) and has a hardness of HV1000 to 3000. On the other hand, the hubs are made from castings (FCD600) which are machined to the dimensions of the parts. The surfaces contacting the tire wheels are lathed to be made generally flat. However, viewed microscopically, fine grooves (stripes) are formed. Further, grooves concentric with the shaft are formed. When such aluminum tire wheels and hubs are fastened, fretting wear occurs. Alumina, which is similar to an abrasive material, is trapped in these grooves. The alumina is successively supplied from the tire wheels. As a result, the tire wheels become thinner leading to fatigue fracture.